The present invention relates to a superconducting electrical machine having a rotor, which is mounted such that it can rotate about an axis of rotation and has a rotor outer housing, which is attached to axial rotor shaft parts and surrounds a winding former having at least one superconducting winding. Furthermore, the rotor has means for mounting the winding former within the rotor outer housing, and on a torque-transmitting side these means comprise a first, rigid connection device between the winding former and the rotor outer housing, and on the opposite side they comprise a second connection device which compensates for changes in axial expansion in the winding former. The second connection device has at least one axial connection element which at one end is connected rigidly to the winding former and whereof the opposite free end forms an axially moving, radially force-fitting connection to at least one centering mounting element of the rotor outer housing. Furthermore, means for cooling and thermally insulating the superconducting winding are provided. A corresponding superconducting electrical machine is known from DE 100 63 724 A1.
Electrical machines, in particular generators or motors, are typically composed of a rotor having a rotating excitation winding and a stator having a fixed stator winding. By using deep-cooled and in particular superconducting conductors, it is possible here to increase the current density and the specific power of the machine, that is, the ratio of power to mass in kilograms, and also to increase the efficiency of the machine.
Generally speaking, deep-cooled windings of electrical machines have to be thermally insulated from the surrounding area and kept at the required cool temperature by a cooling means. Here, effective thermal insulation can only be achieved if the deep-cooled parts of the machine are as far as possible separated from the warm area outside by a high vacuum having a residual gas pressure which is generally below 10−3 mbar, and if connection parts between these deep-cooled parts and the warm area outside transmit as little heat as possible.
Two variants are in particular known for the vacuum insulation of rotors having deep-cooled rotor windings and warm stator windings: in a first construction, the rotor has a warm outer housing and an enclosed vacuum space which rotates with it. In this case, the vacuum space is to surround the deep-cooled region on all sides. However, an undesired transmission of heat to the deep-cooled parts takes place by way of supports which extend through the vacuum space. In a second construction, the substantially cold rotor rotates in a high vacuum. In this case, the outer delimitation of the high vacuum space is defined by the inner bore of the stator. However, an arrangement of this kind requires shaft seals which provide a seal against high vacuum, between the rotor and the stator.
In the first construction mentioned above, which is known for example from DE 23 26 016 B2, the superconducting winding of the rotor is located inside a rotor cryostat which, by means of flanged shafts mounted thereon, forms an outer housing of the rotor. Using conventional superconductor material for the conductors of the winding, helium cooling is provided, giving an operating temperature of approximately 4 K. In addition to the metal superconductor materials which have long been known such as NbTi or Nb3Sn, since 1987 metal oxide superconductor materials with transition temperatures above 77 K have also been known. With conductors using high-Tc superconductor materials of this kind, which are also called HTS materials, it is possible to manufacture superconducting windings of machines that are to be cooled, using liquid nitrogen, to an operating temperature below approximately 77 K. By contrast, the external contour of the rotor outer housing is at approximately room temperature, and in operation may in some cases even be above this.
The net torque of the machine is generated in the rotor winding. The latter is arranged in a cold winding former which, for its part, is suspended or mounted insulated in the rotor outer housing, which acts as a cryostat. In this arrangement, this suspension or mounting on the drive side of the rotor must be stable enough to transmit the torque from the cold winding former to a drive-side shaft part. For this reason, a corresponding, rigid connection device for transmitting torque has to be of relatively solid construction and be connected with force fit to the winding former and the drive-side shaft part. At the same time, this connection device is responsible for centering the cold winding former on the drive side. On the opposite side of the rotor, which is also called the non-drive or operating side because this is where the connections important for operation of the machine such as the supply of cooling means are provided, virtually no torque is taken off. For this reason, substantially only the functions of centering and thermal insulation have to be fulfilled here. However, because when there is a transition from room temperature to operating temperature the axial length of the winding former in relation to the corresponding expansion of the rotor outer housing is reduced by at least one millimeter, the suspension on the operating side must additionally provide the function of a corresponding compensation in length. For this reason, in the prior art disc-shaped connection elements which run radially between the rotor outer housing and the winding former and which enable a corresponding deflection in the axial direction, to compensate for expansion, are provided. As an alternative, it is possible to provide, inside a rotor cryostat, sliding seats which make possible or compensate for axial expansion of the winding former.
A disadvantage of sliding seats inside a rotor cryostat is that the sliding seat is located in the insulating vacuum of the rotor and so the sliding seat cannot be lubricated with lubricants such as oils or greases. As a result of micromovements, for example, on each rotation of the rotor in the machine, the sliding seat of the machine is subject to considerable wear during long-term operation. When the machine is started up, because the rotor cryostat lacks optical transparency, it is moreover not possible to observe the process of shrinkage in the winding former as it cools to a cryogenic temperature. Thus, accurate knowledge of the parameters of the materials and a layout of the sliding seat with sufficient additional play are required to ensure reliable axial mounting of the winding former even at low, cryogenic temperatures. A more spacious layout of the sliding seat and the associated occupation of space result in greater material consumption, higher costs, poorer properties of the machine in operation, and the need for more space for cooling. There is thus less advantage in a more compact construction by comparison with conventional, non-superconducting machines.